1. Field of the Invention
This invention relates to a multi-layer structure polymer composition quite excellent in weather resistance, solvent resistance, stress-whitening resistance, water-whitening resistance and transparency. More particularly, this invention relates to an acrylic multi-layer structure polymer composition not only quite excellent in weather resistance, solvent resistance, stress-whitening resistance and transparency but also in water-whitening resistance which does not readily show the whitening phenomena even if dipped in hot or boiling water. Said composition is a multi-layer structure polymer constituted from an innermost polymer layer (A) consisting mainly of alkyl acrylate and having rather higher crosslink density, a crosslinked elastic polymer (B) consisting mainly of alkyl acrylate and having lower crosslink density than that of innermost polymer layer (A) said innermost polymer layer (A) and said crosslinked elastic polymer (B) constructing a double layer elastomer structure, an outermost polymer (C) consisting mainly of alkyl methacrylate and having a glass transition temperature of 60.degree. C. or higher, and at least one intermediate layer (D) which exists between said polymer (B) layer and said polymer (C) layer and of which alkyl acrylate content linearly decreases from said polymer (B) layer to said polymer (C) layer, and said multi-layer structure polymer has a gel content of at least 50%.
2. Description of the Prior Art
Having a beautiful appearance and a prominent weather resistance, acrylic resins and particularly methyl methacrylate polymer occupy a unique position among the general purpose resins and are in use as a molding material for wide variety of products such as injection-molded products, extrusion-molded products and so on.
On the other hand, since these polymers are hard and brittle, it has been desired to given them an elasticity when they are put to special uses, and various studies have hitherto been conducted for this purpose. As a method for giving them elasticity, a method of blending them with an elastomer, a method of simply copolymerizing them with an elastomeric component a method of graft-polymerizing an elastomer with the component of these resins to give a multi-stage polymer, and various other methods have been employed.
Among the products of these techniques, multi-layer polymers containing acrylic rubber are currently used as molding material for film and sheet having a high weather resistance as characteristic feature. In the existing state of things, however, they lack stress-whitening resistance and water-whitening resistance so that sufficiently satisfactory characteristic properties cannot yet be obtained therefrom. That is, these whitening resistances are very important factors in using them alone as a film or a sheet or as a laminate with other base material, because the whitening at the time of bending, if it occurs, brings about a marked decrease in their commercial value. Multi-layer polymers containing acrylic rubber are proposed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 33,991/77, Japanese Patent Publication No. 3,591/71 and Japanese Patent Publication No. 46,158/74. However, these polymers are products produced by simply multi-layer polymerizing a resin on a crosslinked elastomer, with which it is yet impossible to solve the problem of whitening.
Previously, the present applicant proposed a multi-layer polymer having a tapered structure as a method for solving the above-mentioned problem (Japanese Patent Application Kokai (Laid-Open) Nos. 129,449/76 and 56,150/77). Owing to their unique tapered structure and the effect of grafting, they are impact resistant polymers quite excellent in stress-whitening resistance and in transparency.
These multi-layer polymers having a tapered structure are, indeed, excellent in transparency and stress-whitening resistance. However, they have a fault that they are inferior in the impact resistance at low temperature because they have a resin layer having a high T.sub.g as their core, and the practical problem in the films formed from multi-layer polymers having a tapered structure has consisted in the improvement of their impact resistance at low temperatures, particularly in the temperature range from about 10.degree. C. to about -10.degree. C.
Now, as the new functions obtainable by forming the above-mentioned various acrylic polymers into a film and laminating it with other base material, weather resistance, pollution resistance, decorative property and the like can be referred to. For example, lamination of acrylic film having various printed images is also one of the important uses for exhibiting a decorative effect unobtainable from other base materials themselves.
However, since general acrylic films are readily soluble in or swellable with printing ink solvents such as toluene, methyl ethyl ketone and the like, no clear print can be made on them. That is, they lack solvent resistance.
Regarding this solvent resistance, the films obtained by forming the above-mentioned multi-layer polymers having a tapered structure are similarly unsatisfactory, and their improvement in solvent resistance has been desired. Nevertheless, because of the restriction in their polymer structure, it has been difficult to improve their solvent resistance sufficiently without sacrificing their other properties.
Further, as an important general fault of acrylic films, the readiness with which they undergo whitening when dipped in hot water or boiling water can be referred to. Such phenomena pose a practically important problem particularly in the field of lamination on steel board and fields in which the film is apt to be in contact with hot or boiling water.
No effective method for solving such a problem of whitening phenomena has yet been discovered.